1. Field of the Invention
The present invention relates to a process for the production of hydrophilic coatings on polymeric substrates which are bonded chemically to the surfaces of substrates. The invention furthermore relates to objects with surfaces coated in this way and to their use for industrial, medical or bioengineering purposes.
2. Description of the Background
Polymers (or plastics) with a hydrophilic surface produced by a special treatment have various advantages over non-treated polymers which usually have a hydrophobic surface, depending on the intended use. The higher surface energy has the effect of better wettability with water, aqueous solutions or dispersions and with other liquids of high surface tension. An improved wettability achieved by subsequent hydrophilization, is beneficial or even necessary, for example, if a surface of a plastic is to be dyed or printed with polar dyestuffs or if surfaces of plastics are to be bonded to one another with the aid of a polar adhesive. Fibers and textile fabrics or wovens of polymers also require good wettability for sizing, finishing and dyeing.
Hydrophilization is also of importance for polymeric materials which are used in aqueous systems. Thus, industrial membranes, for example, those used for the desalination of sea water, must be readily wettable in order to be able to display their separating action in full. The surfaces of pipes of plastic or chemical apparatuses must be readily wettable if good heat exchange with the surroundings and/or, in the case of pipes, good flow is required. A good wettability is also of advantage for beds of polymer particles, for example ion exchange resins, and bulk porous layers, for example dialysis membranes, through which material flows. Gas bubbles which settle on the liquid-side surfaces of pipes, hoses or containers of plastic are also undesirable, because they inhibit functioning since the surfaces cannot be wetted adequately by the liquid.
Hydrophilized surfaces of plastic are indispensable for many medical or bioengineering applications, because they are very readily compatible with blood, tissue fluids or other liquids with sensitive biological constituents, in contrast to the standard plastics which are usually hydrophobic by nature, Examples of such applications are blood plasma containers, dialysis hoses, catheters, contact lenses and the like.
The hydrophilization of polymeric substrates can be effected by a single-stage or multistage physical and/or chemical treatment. All known treatment processes have as their objective the provision of hydrophilic groups, such as hydroxyl, carboxyl or also keto groups, on the surface of the polymeric substrate. This can be achieved by processes in which the hydrophilic groups are formed from peripheral layers of the polymer itself. Alternatively or additionally, layers of hydrophilic compounds can be applied to the surface, which may have been treated beforehand, and, if they are vinyl monomers, they can be polymerized.
The single-stage treatment processes which produce the desired hydrophilic groups from the polymer itself include flaming techniques (D. Briggs et al, J. Mater. Sci. 14. 1979, 1344) and corona treatments (J. M. Lane et al, Progress in Organic Coatings 21, 1993, 269-284). However, the hydrophilicity produced as a result of these treatings is often unstable and degrades within hours or days. Plasma processes which produce the hydrophilic groups from the polymer itself in one stage have also been disclosed. According to W. Mohl, Kunststoffe 81 (1981), 7, polyethylene or polypropylene is treated with low pressure plasma and is then more suitable for the production of composite materials. J. F. Friedrich et al in GAK June 1994, Volume 47, 382-388 similarly describe a plasma pretreatment of polymers, for example polyolefins, as a result of which they can be bonded more easily with polyurethanes. Plasma processes produce satisfactory results if the substrates are bonded soon after the treatment. Stabilization of the hydrophilic properties is possible by further reaction, for example with hydrophilic monomers. As a result chemically bonded hydrophilic, optionally bulky groups, which cannot migrate to the inside, are produced on the surface. In addition, plasma processes often lead to erosion of the surface which make the surface rough. This is often undesirable, for example if the objective of the hydrophilization is to reduce the coefficient of friction on contact with water. Roughening of the surface impairs the tribological properties thereof and counteracts this objective.
As a result of a single-stage oxidative treatment with chromium (VI) acid, hydrophilic groups form on the surface of polypropylene from the layers close to the surface (Kang-Wook Lee et al, in Macromolecules 1988, 21, 309-313). Chromium (VI) compounds are avoided where possible in industry, because they are carcinogenic and are not permitted to be discharged into the environment.
In a number of other known processes, the hydrophilic groups are introduced by coating with a hydrophilic coating agent. A distinction can be made here between processes with and without pretreatment of the surfaces of the polymeric substrate, for example, by means of laser, plasma or the like (the initial cleaning of the surface with a solvent, envisaged for almost all the relevant processes, is not rated as a pretreatment).
One coating process without pretreatment of the substrate is the grafting of polypropylene with 2-hydroxyethyl methacrylate (HEMA), which has been described by S. R. Shukla et al in J. Appl. Poly. Sci., Volume 51, 1567-74 (1994). If the polymerization is initiated with UV radiation, the additional use of methanol as a solvent, which is toxicologically unacceptable and pollutes the waste water, is required. If the polymerization is initiated by uranyl nitrate or cerium ammonium nitrate, the heavy metals uranium and cerium must be prevented from entering the waste water.
Coating processes without pretreatment of the substrate also include the procedure disclosed by B. D. Ratner et al, U.S. Pat. No. 5,002,794, in which hydrophilic substances, such as polyethylene glycol or 2-hydroxyethyl methacrylate (HEMA), are deposited on metallic or silicatic surfaces or surfaces of plastic by means of plasma. Hydrophilic monomers, such as HEMA, polymerize spontaneously here under the influence of free radicals formed by the plasma. H. Mirzadeh et al, Biomaterials, 1995, Volume 4, No. 8, 641-648, mention the grafting of acrylamide or HEMA onto a specific polymer, i.e. vulcanized ethylene/propylene rubber, with the aid of a pulsed Co.sub.2 laser. According to S. Edge et al, Polymer Bulletin 27 (1992), 441-445, poly(ether imides) are hydrophilized by photochemical grafting of HEMA from the vapor phase without pretreatment of the surface, using a mercury vapor lamp as the source of radiation. Moreover, according to B. Jansen et al, J. Polymer Sci., Polymer Symposium 66 (1979), 465-473, a specific polyurethane, Tuftan 410 from B. F. Goodrich, can be grafted with HEMA under irradiation with gamma rays from cobalt-60. One disadvantage of this process is the expensive radiation protection measures which it requires.
It remains to be seen whether, in the processes mentioned in the preceding paragraph, the radiation and the plasma effect only polymerization of the monomers or at the same time also activate the surface of the polymeric substrate. The latter is probably the case, since on the one hand, as mentioned above, the hydrophilizing action of the plasma and of the corona treatments on surfaces of plastic is known. At any rate, the radiation and the plasma have such a high energy that the hydrophilic monomers and the resulting polymer are attacked. H. Yasuda accordingly refers, in J. Poly. Sci.: Macromolecular Review, Volume 16, 199-293 (1981), to the undefined and uncontrollable chemistry of plasma polymerization. That molecules are destroyed here can be demonstrated in the coating of surfaces with HEMA by the fact that analysis by ESCA (electron spectroscopy for chemical analysis), according to H. Morra et al, J. Biomed. Mat. Res., 29, 39-45, 1995, gives lower values for oxygen than would be expected from the composition of HEMA and which are also actually found in HEMA polymerized in the customary manner, i.e. by means of free radicals. This may be irrelevant for some applications. However, for medical and bioengineering applications, a layer of intact HEMA is highly desirable, because, as already mentioned, such layers are very readily compatible with the sensitive constituents of the liquids handled in these applications.
However, processes have also been disclosed in which coating with polymerizable monomers is preceded by an activating radiation treatment which modifies the surface of the plastic. Activation and coating of the surface are thus done at separate times. P. Gatenholm et al, Poly. Mater. Sci., 1992, 66, 445-6 describe the hydrophilization of films and microporous membranes of polypropylene by treatment with ozone and subsequent coating with HEMA, polymerization of which is induced by dissociation of the hydroperoxide groups formed on the surface. A disadvantage of this process is that ozone, in a relatively high concentration, destroys the polymer. Finally, H. Thelen et al in Fresenius, J. Anal. Chem. 1995, 353, 290-296 describe a hydrophilizing treatment of polyether sulfones in which the substrate is first treated with nitrogen plasma in the presence of small amounts of oxygen and then coated with HEMK. The process is laborious because the polyether sulfone membrane must be extracted before the coating and, as is also the case in the process of Gatenholm et al, oxygen, which inhibits the polymerization, must be carefully excluded from the HERA solution. Furthermore, the concentration of hydroperoxide groups on the surface and, therefore, the grafting density are difficult to control in the two processes mentioned. A need, therefore, continues to exist for a more effective means of providing a polymeric substrate with a hydrophilic coating.